Is it possible to associate an attribute with a relation? This is required to express for example that Jonh and Ana are spouses since 2014, this date is an attribute of the relationship and not of the entities.
Yes, you are free to do so provided that the relation of interest is allowed to have the attribute of interest.
In your example, provided that the line:
marriage has date;
exists in your schema, you are free to assign an attribute to a given instance by either by referring to the relationship variable:
insert $Ana isa person; $John isa person; $r (spouse: $Ana, spouse: $John) isa marriage; $r has date 2014;
or defining it together with the relationship itself:
insert $Ana isa person; $John isa person; (spouse: $Ana, spouse: $John) isa marriage has date 2014;
as the relationship id unambiguously identifies a relationship instance, provided you know the id, you can also do:
insert $r id ...; $r isa marriage; $r has date 2014;